This invention relates to a magnetic head and a recording/erasing method using the magnetic head, which are usefully applied to electronic still cameras and magnetic recording systems such as magnetic disk recorders for recording photographic images.
An electronic still camera converts an image of a subject to a video signal and records it on a magnetic recording medium such as a floppy disk. For the purpose, the electronic still camera of this kind contains a magnetic head for recording or erasing the video signal on the floppy disk. If a magnetic head for recording and a separate magnetic head for erasing are both contained in the electronic still camera, spaces for the two magnetic heads will be required, which impede designing a compact unit as a portable camera.
To avoid the above disadvantage of using two magnetic heads, there has been proposed a method for using a single magnetic head for both recording and erasing, that is, either a recording current or an erasing current is flown through the coil of a single magnetic head, in order to reduce space.
However, the method using a single magnetic head for recording and erasing has the following new problem.
Since it uses a single magnetic head, the recording track and erasing track have the same widths, but erasing may be made on a different position relative to the recording track, due to deformation or eccentricity of the magnetic recording medium (for floppy disk) or fluctuation in speed (for magnetic tape), resulting in an unerased portion on the track.
The electronic still camera can take normally 50 photos using a field recording method, which uses a floppy disk having 50 annular tracks for recording a signal for one field on each track.
In use of the electronic still cameras, there may be required to increase the recording density, for example, to a double density. To meet such a requirement, the following techniques are generally considered.
(i) The rotation speed of the floppy disk is decreased to a half to shorten the recording wavelengths, thereby increasing the linear recording density. Thus, a track is divided into a first half and a second half, and different field signals are recorded on individual half-tracks. This allows 100 different field signals to be recorded on 50 tracks, thereby providing a double-density recording.
(ii) In addition to the magnetic head having a normal track width, a separate, high-density recording head having a track width of a half that of normal magnetic head is provided. Using the high-density recording magnetic head, 100 tracks are formed to record 100 different field signals, thereby providing a double-density recording.
The conventional art techniques described in (i) and (ii) above have the following problems.
(a) A still video floppy disk has a shortest recording wavelength of 0.5 .mu.m. When the recording wavelength is decreased using the above technique (i), the recording wavelength is near the lower limit of recording wavelength and stable recording cannot be obtained. Moreover, in view of carrier/noise ratio C/N, the technique (i) using the half recording wavelength results in a 6 dB reduced C/N ratio. The value is determined by the following equation (1). EQU Optimum recording point .varies..sqroot.1/4.no.W..lambda. (1)
where
no: number of magnetic particles per unit volume
W: track width
.lambda.: recording wavelength
(b) The technique (ii) is technically realizable, however, it has a disadvantage that an additional high-density magnetic head must be provided, which impedes designing a compact system. When the track width is reduced to a half using the technique (ii), the reduction in C/N radio is only 3 dB, which is advantageous over the technique (i).